1. Field of the Invention
The present invention relates to a DC-DC converter, and particularly, to an oscillator for a DC-DC converter which is able to obtain constant frequency by making output insensitive for changes of voltage and process.
2. Description of the Background Art
Recently, with development of mobile products, DC-DC converter is mainly used for driving controllers or display drivers which control the mobile products, and also, an RC oscillator is mainly used for generating clock of the DC-DC converter.
FIG. 1 is an exemplary view showing a structure of the RC oscillator according to the conventional art.
As shown therein, in the conventional RC oscillator, a resistor (R) and a capacitor (C) are connected in parallel, and a CMOS Schmitt trigger 10 compares high first threshold voltage or low second threshold voltage, which is set in advance, to inputted voltage (Vin) and after that, outputs voltage corresponding to the compared value as inverting it, and feed-forwards the outputted voltage through the resistor R.
FIG. 2 is a detailed circuit diagram showing a structure of the CMOS Schmitt trigger 10 shown in FIG. 1.
As shown therein, the CMOS Schmitt trigger 10 is configured to connect gates of first and second PMOS transistors MP1 and MP2 and gates of first and second NMOS transistors MN1 and MN2 to input side, to connect a contact point between a drain of the first PMOS transistor MP1 and a source of the second PMOS transistor MP2 to a source side of a third PMOS transistor MP3, to connect a contact point between source side of the first NMOS transistor MN1 and a drain of the second NMOS transistor MN2 to which the source is grounded to source side of a third NMOS transistor MN3, to connect a contact point of the drain of the third PMOS transistor MP3 and the drain of the first NMOS transistor MN1 or a contact point of the gate of the third PMOS transistor MP3 and the gate of the third NMOS transistor MN3 to output side after connecting them respectively, and operations of the CMOS Schmitt trigger 10 will be described as follows.
First, high threshold voltage of the CMOS Schmitt trigger 10 is decided by active resistance of the first and third PMOS transistors MP1 and MP3, and the low threshold voltage of the trigger 10 is decided by active resistance of the first and third NMOS transistors MN4 and MN6.
Therefore, the CMOS Schmitt trigger 10 outputs inputted voltage Vin after inverting the voltage in a state that the high and low threshold voltages are decided, and at the same time, is inputted the voltage as much as the voltage drop generated by the capacitor C and the resistor R to process the voltage.
As described above, the RC oscillator mainly consists of the CMOS Schmitt trigger 10, and the CMOS Schmitt trigger 10 is able to obtain the threshold voltage easily by using digital processes in a linear area.
However, when the PMOS and NMOS transistors are used in the linear area, the precise resistance according to the voltages and processes can not be obtained due to the property of the transistor, and therefore, the threshold voltage can be changed from the value set in fabrication process. Therefore, the oscillator is directly affected by the frequency changes, and the RC oscillator can not be used unless the system has a large frequency modulation gain.
Also, the RC oscillator includes the resistor and the capacitor constructed in parallel, and the frequency may be changed by the capacitor, as described above.